J. Nat. Prod. 2006, 69, 43-49
43
Isoflavonoids and Other Compounds from Psorothamnus arborescens with Antiprotozoal Activities Manar M. Salem and Karl A. Werbovetz* DiVision of Medicinal Chemistry and Pharmacognosy, College of Pharmacy, The Ohio State UniVersity, Columbus, Ohio 43210 ReceiVed July 20, 2005
Bioactivity-guided fractionation of the root extract of Psorothamnus arborescens yielded the new isoflavone 5,7,3′,4′tetrahydroxy-2′-(3,3-dimethylallyl)isoflavone (1a) and the new 2-arylbenzofuran 2-(2′-hydroxy-4′,5′-methylenedioxyphenyl)-6-methoxybenzofuran-3-carbaldehyde (2), together with seven known compounds, including three isoflavones, fremontin (3a), glycyrrhisoflavone (4a), and calycosin (5), two pterocarpans, maackiain (6) and 4-hydroxymaackiain (7), one triterpene, oleanolic acid (8), and one chalcone, isoliquiritigenin (9). In addition, the structure of the isoflavone fremontin was revised using spectroscopic and chemical methods and was assigned the new structure 3a. The isoflavone 1a and the chalcone 9 displayed leishmanicidal activity with IC50 values of 13.0 and 20.7 µM, respectively, against Leishmania donoVani axenic amastigotes. Calycosin (5) exhibited selective toxicity against Trypanosoma brucei brucei (IC50 12.7 µM) compared to L. donoVani amastigotes and Vero cells (IC50 100 and 159 µM, respectively). These results prompted us to test a small group of structurally related isoflavones for their antitrypanosomal activities. Genistein and 7,3′,4′-trihydroxyisoflavone displayed promising activity (IC50 values 4.2 and 7.1 µM, respectively) and selectivity (IC50 versus Vero cells: 32.9 and 135 µM, respectively). These studies suggest that the isoflavone skeleton deserves further investigation as a template for novel antileishmanial and trypanocidal compounds. The genus Psorothamnus (name from the Greek psoros and thamnus, meaning scurfy or scab shrub) belongs to the legume family (Fabaceae). It was shown in our laboratory to be a rich source of antiparasitic compounds.1 The genus includes nine species found in the deserts of southwestern North America.2 Previously, we isolated antileishmanial and trypanocidal compounds from P. polydenius (S. Watson) Rydberg that displayed selectivity for these organisms.1 The plant P. arborescens (Torrey ex A. Gray) Barneby var. minutifolius (Parish) Barneby, Fabaceae-Papilionoideae2,3 [synonym Dalea fremontii Torrey ex A. Gray var. minutifolia (Parish) L. Benson]4 is also known as Mojave dalea5 or indigo bush.6 This plant is a small, thorny desert shrub up to 1 m in height with deep indigo blue or violet flowers and is locally common through the west-central and north Mojave Desert, California, growing chiefly on granitic and volcanic bedrock.2,5 This species has not been investigated before, and there are no previous reports of any antiparasitic activity associated with this plant. The closely related P. fremontii (D. fremontii), which occurs in California only on the sedimentary ranges of the far eastern Mojave Desert,2 is reported to have been used by the indigenous tribes to stop internal hemorrhage and for the treatment of stomach problems.7 P. arborescens extracts exhibited significant activity against Leishmania donoVani axenic amastigotes and bloodstream form Trypanosoma brucei brucei. Bioassay-guided fractionation of the ethanolic root extract from this plant resulted in the isolation of several bioactive compounds. In this paper, we report on the antiprotozoal activities of compounds 1a-9 and the structure elucidation of compounds 1a and 2. The structure of fremontin, an isoflavone previously isolated from P. fremontii,8 has also been revised.
Results and Discussion Bioactivity-guided fractionation of the ethanolic extract of P. arborescens yielded four isoflavones, the new 5,7,3′,4′-tetrahydroxy2′-(3,3-dimethylallyl)isoflavone (1a), fremontin (3a),8 glycyrrhisoflavone (4a),9 and calycosin (5),10-13 a new 2-arylbenzofuran, 2-(2′-hydroxy-4′,5′-methylenedioxyphenyl)-6-methoxybenzofuran3-carbaldehyde (2), the pterocarpans maackiain (6)14-17 and 4-hy* To whom correspondence should be addressed. Tel: (614) 292-5499. Fax: (614) 292-2435. E-mail:
[email protected]. 10.1021/np0502600 CCC: $33.50
droxymaackiain (7),17,18 the triterpene oleanolic acid (8),10,19 and the chalcone isoliquiritigenin (9).20,21 The structures of the known compounds were determined by 1D and 2D NMR techniques and confirmed by comparing the physical and spectroscopic/spectrometric data with those from the literature (NMR and MS). Compound 1a isolated from P. arborescens had a 1H NMR spectrum (see Table 1) showing a hydrogen-bonded OH group at δ 13.00, a downfield singlet at δ 7.89, two o-coupled aromatic protons at δ 6.56 and 6.76 (J ) 8.0 Hz), two m-coupled aromatic protons at δ 6.29 and 6.43 (J ) 1.3 Hz), and a methylene group at δ 3.30 displaying 1H-1H COSY correlation to a downfield methine at δ 5.08, in addition to two methyls at δ 1.43 and 1.52. The 13C and DEPT NMR spectra showed a carbonyl at δ 182.0, six oxygenated carbons in the aromatic region at δ 165.0, 163.8, 159.2, 155.2, 145.6, and 144.2, and three methines at δ 124.4, 123.0, and 113.1, in addition to two relatively upfield methines at δ 99.8 and 94.5, one methylene at δ 27.3, two methyls at δ 25.6 and 17.6, and five quaternary carbons at δ 130.8, 129.2, 125.3, 123.9, and 106.1. HRESIMS confirmed the deduced molecular formula (measured [M + Na]+ 377.1000, calcd 377.1001 for C20H18NaO6). The NMR and MS data indicated an isoflavone structure with a 5,7-dihydroxylated A-ring and a 2′,3′,4′-trisubstituted B-ring with two hydroxyls and one isoprenyl group. This was supported by an HMBC correlation between H-2 (δ 7.89) and C-1′, which is correlated to H-5′ and H-1′′. H-6′ showed three-bond HMBC correlations to C-3 and C-2′. H-1′′ and H-2′′ of the isoprenyl group correlated to C-2′, thus establishing the position of the isoprenyl group at C-2′. Further confirmation was provided via a selective NOE experiment in which H-2 showed NOE correlations to H-6′, H-1′′, and H-2′′. Assignment of ring B oxygenated carbons is based on the HMBC correlation of H-1′′ to C-3′ and that of H-6′ to C-4′. Furthermore, acid-catalyzed cyclization of the 2′-isoprenyl group gave the dihydrobenzopyran derivative 1b, verifying the presence of an OH group at the 3′ position. Further evidence against the presence of an OH group in the 2′ position is the relatively downfield position of the OH-5 signal (δ 13.0); a 2′-OH group in an isoflavone structure would be expected to exert a significant shielding effect on the 5-OH, resulting in a more upfield chemical shift (δ 12.51-12.79).22,23 Thus compound 1a was identified as the new 5,7,3′,4′-tetrahydroxy-2′-(3,3-dimethylallyl)isoflavone. The isoflavanone arizonicanol D, isolated from the root of Sophora
© 2006 American Chemical Society and American Society of Pharmacognosy Published on Web 12/22/2005
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Journal of Natural Products, 2006, Vol. 69, No. 1
Salem and WerboVetz
Chart 1
Table 1. 1H and 13C NMR Data and HMBC Correlations for Compounds 1a and 1b in Acetone-d6 1a position
δC
2 3 4 5 6 7 8 9 10 1′ 2′ 3′ 4′ 5′ 6′ 1′′ 2′′ 3′′ 4′′ 5′′ 5-OH
155.2 125.3 182.0 163.8 99.8 165.0 94.5 159.2 106.1 123.9 129.2 144.2 145.6 113.1 123.0 27.3 124.4 130.8 17.6 25.6
a
δH (mult.; JHH, Hz)
1b HMBC
7.89 (s)
C-3, 4, 9, 10a,1′
6.29 (d; 1.3)
C-4a, 5, 7, 8, 10
6.43 (d; 1.3)
C-4a, 6, 7, 9, 10
6.76 (d; 8.0) 6.56 (d; 8.0) 3.30 (d; 6.0) 5.08 (m)
C-1′, 2′a, 3′ C-3, 2′, 4′, 1′′a C-1′, 2′, 3′, 2′′, 3′′ C-2′, 1′′, 4′′, 5′′
1.43 (s) 1.52 (s) 13.00 (s)
C-2′′, 3′′, 5′′ C-2′′, 3′′, 4′′ C-5, 6, 10
δC 155.5 124.9 181.5 163.9 99.9 165.1 94.6 159.3 106.0 122.9b 122.7b 142.4 147.5 112.7 122.8 21.3 33.3 75.2 26.8 26.8
δH (mult.; JHH, Hz)
HMBC
8.02 (s)
C-3, 4, 9, 1′
6.30 (d; 1.3)
C-4a, 5, 7, 8, 10
6.44 (d; 1.3)
C-4a, 6, 7, 9, 10
6.70 (d; 8.0) 6.64 (d; 8.0) 2.66 (br t; 6.8) 1.78 (t; 6.8)
C-1′, 3′ C-3, 2′, 4′ C-1′, 2′, 3′, 2′′, 3′′ C-2′, 1′′, 3′′, 4′′, 5′′
1.35 (s) 1.35 (s) 12.98 (s)
C-2′′, 3′′, 5′′ C-2′′, 3′′, 4′′ C-5, 6, 10
Weak four-bond HMBC correlations. b Entries with the same superscript are interchangeable.
arizonica (Fabaceae), possesses a B-ring structure similar to that found in compound 1a, and their 1H and 13C NMR data are in agreement.24 The structure of compound 1a is also closely related to that of the isoflavone piscidone isolated from the root bark of the Jamaican dogwood Piscidia erythrina (Fabaceae).23
The new 2-arylbenzofuran 2 exhibited a 1H NMR spectrum (see Table 2) showing a downfield singlet at δ 10.00, a 1,2,4trisubstituted benzene ring with protons at δ 7.50 (d, J ) 8.4 Hz), 6.66 (dd, J ) 2.2 and 8.4 Hz), and 6.71 (J ) 2.2 Hz), two isolated aromatic singlets at δ 7.52 and 7.16, a downfield methylene singlet
IsoflaVonoids from Psorothamnus arborescens
Journal of Natural Products, 2006, Vol. 69, No. 1 45
Table 2. 1H and 13C NMR Data and HMBC Correlations for Compound 2 in Acetone-d6 position
δC
2 3 4 5 6 7 8 9 1′ 2′ 3′ 4′ 5′ 6′ 3-CHO 6-OCH3 -OCH2O-
163.5 118.3 133.5 108.9 159.9 100.5 162.5 109.8 119.5 150.4 94.1 147.8 146.7 100.9 187.7 56.1 102.7
δH (mult.; JHH)
HMBC
7.50 (d; 8.4) 6.66 (dd; 2.2,8.4)
C-6, 8 C-6, 7, 9
6.71 (d; 2.2)
C-5, 6, 8, 9
7.16 (s)
C-1′, 2′, 5′
7.52 (s) 10.00 (s) 3.86 (s) 6.08 (s)
C-2, 2′, 4′ C-3 C-6 C-4′, 5′
at δ 6.08 indicating a methylenedioxy group, and a methoxy group at δ 3.86. The 13C and DEPT NMR spectra showed a carbonyl signal at δ 187.7, six oxygenated aromatic carbons at δ 163.5146.7, five aromatic methines and three quaternary carbons at δ 133.5-94.1, one methylenedioxy group at δ 102.7, and one methoxy group at δ 56.1. The deduced molecular formula C17H12O6 was confirmed by HRESIMS (measured [M + Na]+ 335.0550, calcd 335.0532). The carbonyl signal at δ 187.7 showed HSQC correlation to the proton singlet at δ 10.00, thus indicating a carbaldehyde group that was assigned to position 3 on the basis of an HMBC correlation between the aldehydic proton and C-3. The position of the methoxy group was established by an HMBC correlation to C-6 and a NOESY correlation to H-5 and H-7. The methylenedioxy protons showed HMBC correlations to the ooxygenated carbons of ring C (δ 146.7 and 147.8). The aromatic singlet at δ 7.52 was assigned to H-6′ on the basis of its threebond HMBC correlations to C-2, while that at δ 7.16 to H-3′ on the basis of three-bond correlations to C-1′ and 5′. Thus compound 2 was shown to be 2-(2′-hydroxy-4′,5′-methylenedioxyphenyl)-6methoxybenzofuran-3-carbaldehyde. The compound closest in structure to 2 is cicerfuran isolated from the wild chickpea Cicer bijugum.25
Figure 1. Fremontin: literature and proposed structures. The 1H and 13C NMR data (Table 3) as well as the physical data for compound 3a closely matched those reported by Manikumar et al.8 for the isoflavone fremontin (see Figure 1) isolated from the related species P. fremontii. However, the 13C NMR data showing ring B oxygenated carbons at relatively upfield positions (δ 143.1 and 147.3) seemed more consistent with the OH groups in ortho rather than meta positions to each other; oxygenated carbons with no ortho/para oxygenation are expected to be more downfield (δ 155-165).26 Furthermore, the OH-5 signal in acetoned6 occurred at δ 13.00, a more downfield value than that expected for a 2′-hydroxylated isoflavone (δ 12.51-12.79),22,27 implying that position 2′ is not hydroxylated. Tahara et al. suggested that the structure of fremontin should be reexamined on the basis of the OH-5 chemical shift22 and later proposed a possible B-ring structure with two ortho-OH groups at positions 3′ and 4′ and the isoprenyl group assigned to position 6′.28 Our HMBC data (see Table 3) supported the proposed structure (Figure 1) where correlations of H-2′ at δ 6.56 with C-3 and C-6′, and H-5′ at δ 7.02 with C-1′ and -3′, were observed. To shed further light on the structure of this compound, we performed selective NOE experiments to irradiate the methyl group at δ 1.33, resulting in enhancement of the H-2 and the H-5′ singlets by 0.1 and 0.5%, respectively. Alternatively, irradiating the H-2 singlet enhanced the signals of the nearby isoprenyl methyl groups in the 6′ positions of ring B by 0.5 and 0.6%, while irradiating H-5′ strongly enhanced the isoprenyl methyls by 9.1 and 3.1%. In addition, acetylation of fremontin gave the tetraacetyl derivative 3b, whose 1H NMR signals for H-2′ and H-5′ experienced a significant downfield shift of 0.39 and 0.37 ppm, respectively, indicating that both protons are ortho to the acetylated OH group(s).11 Since those protons are in the para position to each other, it follows that the only arrangement possible is that of the structure 3b. Additional evidence came from the trimethylated
Table 3. 1H and 13C NMR Assignments and HMBC Correlations for Compounds 3a (acetone-d6) and 3c (CDCl3) 3a position 2 3 4 5 6 7 8 9 10 1′ 2′ 3′ 4′ 5′ 6′ 1′′ 2′′ 3′′ 4′′ 5′′ 5-OH 3′-OCH3 7′-OCH3 4′-OCH3 a
δC 155.0 126.8 183.0 163.7 99.7 165.1 94.4 159.1 105.9 122.2 121.1 143.5 145.7 115.4 140.2 42.2 150.0 109.4 29.4 30.0
δH (mult.; JHH,, Hz)
3c HMBC
δC
7.79 (s)
C-3, 4, 9, 1′
6.27 (d; 1.9)
C-5, 7, 8, 10
6.39 (d; 1.9)
C-6, 7, 9, 10
6.56 (s)
C-3, 3′,4′, 6′
7.02 (s)
C-1′, 3′, 4′, 1′′
153.6 126.1 182.1 162.6 98.1 165.5 92.4 158.0 106.0 121.6 115.7 146.9 148.7 111.2 140.3 42.0 148.5 109.8
6.01 (dd; 10.6, 17.6) 4.77 (dd; 1.2, 17.6) 4.64 (dd; 1.2, 10.6) 1.29 (s) 1.33 (s) 13.01 (s)
Assignments are interchangeable.
C-6′, 1′′, 4′′, 5′′ C-1′′, 2′′ C-1′′ C-6′, 1′′, 2′′, 5′′ C-6′, 1′′, 2′′, 4′′ C-5, 6, 10
28.9 29.7 55.78a 55.84a 55.94a
δH (mult.; JHH,, Hz)
HMBC
7.59 (s)
C-3, 4, 9, 1′
6.38 (d; 2.1)
C-5, 7, 8, 10
6.40 (d; 2.1)
C-6, 7, 9, 10
6.54 (s)
C-3, 3′, 4′, 6′
7.06 (s)
C-1′, 3′, 4′, 6′, 1′′
6.02 (dd; 10.6, 17.5) 4.81 (d; 17.5) 4.73 (d; 10.6) 1.37 (s) 1.40 (s) 12.81 (s) 3.83 (s) 3.88 (s) 3.92 (s)
C-1′′, 4′′, 5′′ C-1′′, 2′′ C-1′′, 2′′ C-6′, 1′′, 2′′, 5′′ C-6′, 1′′, 2′′, 4′′ C-5, 6, 10 C-3′ C-7 C-4′
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Journal of Natural Products, 2006, Vol. 69, No. 1
derivative 3c, prepared by reacting fremontin with dimethyl sulfate in the presence of K2CO3, followed by separating the tri- (3c) and tetramethylfremontin (3d) products. The OMe groups in 3c at δ 3.83, 3.88, and 3.92 were assigned at the 3′, 7, and 4′ positions, respectively, due to HMBC correlations (see Table 3) with C-3′, -7, and -4′, respectively. Irradiation of H-5′ at δ 7.06 resulted in enhancement of the two isoprenyl methyl signals by 3.4 and 4.1%, and only the 4′-OMe group at δ 3.92 by 6.5%, while irradiating the H-2′ at δ 6.54 resulted in enhancing the 3′-OMe peak at δ 3.83 by 5.6% and the H-2 signal at δ 7.59 by 0.5%. On the other hand, irradiating the H-2 proton at δ 7.59 resulted in enhancing the H-2′ signal at δ 6.54 (0.5%) and the isoprenyl group methyls at δ 1.37 and 1.40 by 0.5 and 0.7%. Glycyrrhisoflavone (4a), which is closely related in structure to compound 1a, showed 1H and 13C NMR spectra consistent with those reported in the literature.9 However, we assigned the following signals differently due to the current availability of HMBC data. The peak at δ 6.84 is assigned to H-6′ on the basis of its threebond correlation to C-3 and C-1′′, while the peak at δ 7.02 is assigned to H-2′, as it shows correlation to C-3, -4′, and -6′. C-5′ is assigned to δ 128.8 on the basis of its correlation to H-2′′. Further support for the structure was obtained in an NOE experiment where the H-2 was irradiated, resulting in enhancement of the H-6′ and to a lesser extent H-2′. The cyclized (4b) and methylated (4c) derivatives were prepared for biological testing. The NMR, UV, and IR data of maackiain (6) were in agreement with the literature values.15-17 Despite the presence of two chiral centers in the structure, only the 6aR,11aR and 6aS,11aS cisconfigurations are sterically possible.29 However, the specific rotation data did not completely match those reported for either the pure isomers or the racemate. The specific rotation is negative ([R]D24 ) -33.5, CHCl3), but the absolute value is much less than that reported for the optically pure (-)-isomer ([R]D ) -251.7, CHCl3).14 This suggests that the compound isolated is a mixture of (-)-6aR,11aR-maackiain and the (+)-6aS,11aS-isomer with an excess of the (-)-form. The same situation is true for the isolated 3,4-dihydroxy-8,9-methylenedioxypterocarpan (7), which is a 4-hydroxy analogue of maackiain (6). The specific rotation of compound 7 in CHCl3, [R]D24 ) +62.2, is less than that reported for the (+)isomer in the same solvent ([R]D ) +154). This, again, suggests the presence of a mixture of (-)-6aR,11aR-4-hydroxymaackiain and the (+)-6aS,11aS-isomer with an excess of the (+)-form. The activities of the isolated compounds against L. donoVani axenic amastigotes and T. b. brucei, as well as against two mammalian cell lines (Vero cells and PC-3 prostate cancer cells), are summarized in Table 4. Compounds 1a and 9 showed significant antileishmanial and trypanocidal activities, displaying IC50 values below 21 µM. However, they showed low selectivity (around 3-fold) for the parasites over the mammalian Vero cells. The selectivity is worse for pterocarpan 7, which shows higher toxicity toward Vero cells than to the parasites. On the other hand, calycosin (5) displayed selective toxicity (about 15-fold) toward T. b. brucei bloodstream forms (IC50 12.7 µM) over Vero cells. Calycosin is reported to have antiplasmodial,30,31 antileishmanial,32 and antigiardial effects.33 In addition, calycosin was found to protect human umbilical vein endothelial cells from hypoxia-induced impairment.34 Methylation of the isoflavones 3a and 4a moderately improved the trypanocidal but not the antileishmanial activity. Tetramethylglycyrrhisoflavone (4c) was 3.7-fold more active toward trypanosomes than the parent glycyrrhisoflavone (4a). Interestingly, compound 2 did not show any antikinetoplastid activity in our assays. Related Fabaceaederived 2-arylbenzofurans were reported to possess antifungal25 and antimalarial30 activities. The trypanocidal selectivity of calycosin prompted us to assay a group of related natural isoflavones that are commercially available (see Figure 2); the results are summarized in Table 5. Genistein, a common dietary isoflavone (especially in soy-based diets), was the most potent against
Salem and WerboVetz
Table 4. Bioactivity of Compounds 1a-9 against Axenic L. donoVani, T. b. brucei, and Two Mammalian Cell Lines IC50 (µM)a compound
L. donoVani axenic T. b. brucei amastigotes variant 221
1a 1b 2 3a 3b 3c 3d 4a 4b 4c 5 6 7 8 9 ansamitocin P3 pentamidine podophyllotoxin suramin
Vero cells
PC-3 cells
37.5 ( 10.2 NTb >250 134 ( 27 NT NT NT 52.8 ( 11.3 NT 44.3 ( 7.3 159 ( 17 >250 3.3 ( 1.7 >250 65.6 ( 19.5 1.2 ( 0.3c NT NT
12.7 ( 3.1 107 ( 16 >250 78.5 ( 27.4 66.2 ( 28.5 146 ( 4 134 ( 30 62.1 ( 7.3 79.3 ( 9.6 30.9 ( 4.6 192 ( 29 >250 28.0 ( 10.0 211 ( 10 46.4 ( 7.8 NT NT 12.5 ( 1.1c
13.0 ( 0.8 38.4 ( 6.2 >250 123 ( 4 42.7 ( 1.5 >250 86.0 ( 11.0 46.3 ( 2.5 68.3 ( 6.5 41.4 ( 2.7 100 ( 10 >250 37.3 ( 1.0 >250 20.7 ( 2.3 NT 2.3 ( 0.4 NT
12.1 ( 1.9 57.9 ( 2.5 >250 75.3 ( 5.9 67.8 ( 5.6 32.3 ( 2.0 41.7 ( 12.2 33.0 ( 2.5 77.0 ( 3.4 7.8 ( 2.2 12.7 ( 0.7 130 ( 13 3.7 ( 0.7 43.1 ( 8.3 32.8 ( 2.0 NT NT NT
NT
0.19 ( 0.03 NT
NT
Values represent the mean ( standard deviation of at least 3 independent experiments. b NT: not tested. c IC50 values in nM. a
Figure 2. Structures of the additional compounds tested for antiprotozoal activity. Table 5. Bioactivity of Selected Isoflavones against Axenic L. donoVani, T. b. brucei, and Vero Cells IC50 (µM) compound genistein biochanin A 7-hydroxyisoflavone formononetin prunetin daidzein 7,4′-dimethoxyisoflavone 7,3′,4′-trihydroxyisoflavone apigenin pentamidine suramin
L. donoVani axenic amastigotes 73.0 ( 5.6 89.9 ( 1.8 113 ( 6 NT NT NT NT NT NT 1.5 ( 0.2 NT
T. b. brucei variant 221
Vero cells
4.2 ( 0.5 12.3 ( 1.0 94.2 ( 4.0 90.2 ( 14.1 20.2 ( 4.3 23.7 ( 5.0 >100 7.1 ( 1.6 11.9 ( 0.9 NT 0.183 ( 0.025
32.9 ( 8.4 NTb NT NT NT NT NT 135 ( 13 NT NT NT
a Values represent the mean ( standard deviation of at least 3 independent experiments. b NT: not tested.
trypanosomes. This compound displayed an IC50 of 4.2 µM, making it 3-fold more active than calycosin (IC50 of 12.7 µM) on a molar basis. Past reports have shown that genistein inhibits trypanosomal protein tyrosine kinase (PTK) activity.35 Later, it was shown that genistein is an efficient inhibitor of trypanosomal protein synthesis and phosphorylation (which is primarily serine and threonine phosphorylation in these organisms), indicating that the antitrypanosomal effect of this compound is not due to a specific effect on the tyrosine kinase activity.36 Daidzein, a genistein analogue that is known to be inactive against PTK,37 still displayed an inhibitory effect (IC50 of 23.7 µM) against the parasites. Genistein was also reported to show activity against both chloroquine-sensitive and -resistant Plasmodium falciparum.31 7,3′,4′-Trihydroxyisoflavone
IsoflaVonoids from Psorothamnus arborescens
also showed a significant antitrypanosomal effect (IC50 of 7.1 µM), while being virtually nontoxic to Vero cells. Apigenin, a flavone with the same substitution pattern as that of genistein, was about 3-fold less active than genistein against T. b. brucei. Our studies with compounds from P. arborescens and related commercially available molecules revealed several isoflavonoids with interesting antiprotozoal activities. These results indicate that the isoflavone skeleton is worthy of further investigation as a template for novel antitrypanosomal compounds. Experimental Section General Experimental Procedures. Melting points were measured on a Thomas-Hoover capillary melting point apparatus and were uncorrected. Optical rotations were determined on a Perkin-Elmer polarimeter using a 100 mm glass microcell. UV-vis spectra were taken in methanol using a SPECTRAmax PLUS spectrophotometer (Molecular Devices, Sunnyvale, CA). IR spectra were obtained in KBr on a Nicolet Prote´ge´ 460 FT-IR spectrophotometer. Nuclear magnetic resonance (NMR) spectra were obtained on Bruker 300 and 400 MHz spectrometers using the solvents CDCl3, CD3CN, methanol-d4, or acetone-d6 (Aldrich) with TMS as the internal standard. 13C DEPT, 1 H-1H COSY, DPFGSE-NOE (selective NOE),38 NOESY, HSQC, and HMBC NMR spectra were obtained using standard Bruker pulse sequences. All accurate mass experiments were performed on a Micromass ESI-Tof II (Micromass, Wythenshawe, UK) mass spectrometer. Column chromatography was conducted using silica gel 60 (63-200 µm particle size) from EM Science or Sephadex LH-20 from Amersham Biosciences. Silica gel 60 H (15-45 µm particle size) from EM Science was used for vacuum-liquid chromatography (VLC). Precoated TLC silica gel 60 F254 plates from EM Science were used for thin-layer chromatography (0.25 and 2 mm layer thickness for analytical and preparative TLC, respectively). Spots were visualized using UV light or vanillin/sulfuric acid reagent. HPLC runs were carried out using a System Gold model 127 pump equipped with a model 166 UV detector (Beckman) and 4.6 × 250 mm or 10 × 250 mm C18-A Polaris columns (Varian) for analytical (flow rate ) 1 mL/min) or semipreparative runs (flow rate 5 mL/min), respectively. The solvents used were H2O, MeOH, and CH3CN, each with 0.1% HOAc, and are designated as A, B, and C, respectively. Daidzein, 7,4′-dimethoxyisoflavone, 7,3′,4′-trihydroxyisoflavone, and apigenin were purchased from Alfa Aesar and genistein from Indofine Chemical Company (Somerville, NJ). Other compounds and reagents were obtained from Sigma-Aldrich. Plant Material. P. arborescens var. minutifolius root was collected on June 11, 2003, from E. Tulare Co., California, on the eastern slopes of the southern Sierra Nevada in desert-chaparral transitional vegetation by Dr. Richard Spjut (WBA 4841-13, SPJ-15357), World Botanical Associates (Bakersfield, CA). Voucher specimens are deposited at the Botanical Research Institute of Texas (BRIT), the United States National Herbarium, the Smithsonian Institution (US), and the World Botanical Associates (WBA). Extraction and Isolation. The dried and powdered root material (0.6 kg) was percolated with 2.9 L of 95% EtOH. The resulting gummy extract (38 g) was partitioned between H2O and CH2Cl2. The organic layer was then partitioned between 90% MeOH (F001, 12.0 g) and hexane (F002, 2.0 g). The aqueous fraction was further partitioned between EtOAc (F003, 0.5 g) and H2O (F004, 10.4 g). The activity was concentrated in F001 (IC50 ) 16.5 µg/mL in the axenic amastigote assay), which was chromatographed over Sephadex LH-20 eluting with MeOH. The fractions were combined according to their TLC profile into seven pools (F006-F012). The bioactivity was found to be highest in fractions F008, F009, and F010. F008 (0.86 g) was chromatographed on a silica gel column eluting with a gradient of 20% EtOAc-hexane to 100% EtOAc. The fraction eluted with 25% EtOAc-hexane was purified using preparative HPLC with an isocratic solvent system of 90% B in A, λmax 210 nm, to collect 23 mg of compound 8 (tR 11 min). F009 (9.4 g) was subjected to silica gel chromatography using a solvent gradient from 30% EtOAc-hexane to 100% EtOAc to 25% MeOH-EtOAc to obtain nine fractions, F009-1 to F009-9. F009-4 (750 mg) was subjected to Sephadex LH-20 chromatography followed by HPLC using a gradient from 50% to 60% B in A over 30 min then isocratic 60% B in A to yield compound 2 (3.5 mg, tR 37 min),
Journal of Natural Products, 2006, Vol. 69, No. 1 47
compound 4 (22.5 mg, tR 53 min), and crude compound 9 (20 mg, tR 8-12 min). The latter was purified by additional HPLC using a gradient solvent of 40% to 60% B in A in 60 min to isolate 9.1 mg of 9 (tR ) 48-54 min). F009-5 (500 mg) was chromatographed on a silica gel column using a gradient solvent of 4% EtOAc-CHCl3 to 100% CHCl3 to 5% MeOH-CHCl3. Fractions were combined on the basis of their TLC profile to give 150 mg of crude compound 1a, which was purified by HPLC using a solvent gradient of 50% B in A increased to 60% over 30 min then kept isocratic at 60% B, collecting fractions eluting at tR 28-34 min to give 94 mg of pure compound 1a. F009-8 (917 mg) was subjected to silica gel chromatography eluting with a gradient of 100% CH2Cl2 to 100% EtOAc. Additional silica gel column chromatography of the fraction was performed eluting with 35% EtOAc in CH2Cl2 (F009-8-3, 260 mg) using the isocratic solvent CHCl3MeOH-H2O (90:6:1, lower phase) followed by HPLC of subfraction 3 to obtain compound 5 (6 mg, isocratic 20% C in A, tR 32 min) and subfraction 4 (50% B in A increased to 58% over 24 min, then kept isocratic, collecting fractions eluting at tR 24-28 min) to give 42.6 mg of compound 3a. A portion (95 mg) of F010 was purified using preparative HPLC with an isocratic solvent system of 50% solvent B in A to obtain 24 mg of compound 7 (tR 13.5 min) and 57 mg of compound 6 (tR 29 min). 5,7,3′,4′-Tetrahydroxy-2′-(3,3-dimethylallyl)isoflavone (1a): greenishwhite residue; UV (MeOH) λmax (log ) 210 (4.53), 260 (4.42), 285 sh (4.00) nm; IR νmax (KBr) 3446, 3223 (br OH), 3081, 2976, 2927, 1704, 1654, 1619, 1570, 1492, 1444, 1360, 1304, 1200, 1051 cm-1; 1H, 13C (acetone-d6, 300 and 75 MHz, respectively), and HMBC NMR data, see Table 1; HRESIMS m/z 377.1000 [M + Na]+ (calcd for C20H18NaO6, 377.1001). Acid-Catalyzed Cyclization of Compound 1a. Compound 1a (9.5 mg, 0.03 mmol) was stirred with 88% HCOOH (1 mL) at 80 °C overnight. The product 1b (8.0 mg, 84% yield) was purified by HPLC (50 to 100% solvent B in A in 50 min; tR ) 22 min; mp 247-249 °C; 1 H, 13C (acetone-d6, 400 and 100 MHz, respectively), and HMBC NMR data, see Table 1). 2-(2′-Hydroxy-4′,5′-methylenedioxyphenyl)-6-methoxybenzofuran3-carbaldehyde (2): white residue; UV (MeOH) λmax (log ) 247 (4.21), 295 (3.93), 346 (4.05) nm; IR νmax (KBr) 3281 (br OH), 2867, 1656, 1618, 1589,1506, 1461, 1314, 1202, 1141, 1034, 950, 893, 802 cm-1; 1 H, 13C (acetone-d6, 400 and 100 MHz, respectively), and HMBC NMR data, see Table 2; HRESIMS m/z 335.0550 [M + Na]+ (calcd for C17H12NaO6, 335.0532). Fremontin (3a): greenish-white powder; UV (MeOH) λmax (log ) 259 (4.37), 292 (3.99) nm; IR νmax (film) 3366 (br OH), 1652, 1617, 1576, 1508, 1437, 1361, 1277, 1169 cm-1; 1H, 13C (acetone-d6, 400 and 100 MHz, respectively), and HMBC NMR data, see Table 3; HRESIMS m/z 377.1027 [M + Na]+ (calcd for C20H18NaO6, 377.1001). Acetylation of Fremontin. Fremontin (4 mg, 0.01 mmol) was acetylated with acetic anhydride (0.3 mL) in pyridine (0.3 mL) by stirring at room temperature overnight. Fremontin tetraacetate (3b, 3.5 mg, 59% yield) was purified by HPLC (gradient B in A, 70 to 100% in 30 min; tR ) 7.5 min; mp 195-197 °C (lit.8 mp 195 °C); 1H NMR (acetone-d6, 400 MHz) δ 8.00 (1H, s, H-2), 7.39 (1H, s, H-5′), 7.35 (1H, d, J ) 2.2 Hz, H-8), 6.97 (1H, d, J ) 2.2 Hz, H-6), 6.95 (1H, s, H-2′), 5.99 (1H, dd, J ) 10.6, 17.5 Hz, H-2′′), 4.83 (1H, dd, J ) 1.1, 17.5 Hz, H-3′′a), 4.72 (1H, dd, J ) 1.1, 10.6 Hz, H-3′′b), 2.34, 2.29, 2.26, and 2.25 (3H each, s, COCH3), 1.38 and 1.33 (3H each, s, CH34′′ and -5′′); 13C NMR (acetone-d6, 100 MHz) δ 175.9 (C, C-4), 169.3, 168.8, 168.7, and 168.5 (C each, COCH3), 158.6 (C, C-9), 155.2 (C, C-7), 153.6 (CH, C-2), 151.6 (C, C-5), 148.7 (CH, C-2′′), 147.3 (C, C-4′), 143.1 (C, C-3′), 141.0 (C, C-6′), 130.2 (C, C-1′), 128.9 (CH, C-2′), 128.3 (C, C-3), 123.3 (CH, C-5′), 116.1 (C, C-10), 114.8 (CH, C-6), 110.8 (CH2, C-3′′), 109.9 (CH, C-8), 42.8 (C, C-1′′), 29.9 and 29.5 (CH3 each, CH3-4′′ and -5′′), 21.0, 21.0, 20.53, and 20.45 (CH3 each, COCH3); HMBC correlations: H-2 to C-3, -4, -9, -1′, H-6 to C-5, -7, -8, -10, H-8 to C-6, -7, -9, -10, H-2′ to C-3, -3′, -4′, -6′, H-5′ to C-1′, -3′, -6′, -1′′, H-2′′ to C-1′′, -4′′, -5′′, H-3′′ to C-1′′, -2′′, CH34′′ to C-6′, -1′′, -2′′, -5′′, CH3-5′′ to C-6′, -1′′, -2′′, -4′′, COCH3 to COCH3. Methylation of Fremontin. Fremontin (20 mg, 0.06 mmol) in anhydrous acetone (9 mL) was refluxed for 2 h with excess K2CO3 (125 mg) and Me2SO4 (0.1 mL, 1.06 mmol) with molecular sieves (4 Å). PTLC (2% MeOH in CHCl3) of the products yielded trimethylfremontin (3c, 10.5 mg, 47% yield), which was recrystallized from EtOAc-hexane as grayish needles (Rf 0.47; mp 163-164 °C; 1H, 13C
48
Journal of Natural Products, 2006, Vol. 69, No. 1
(CDCl3, 400 and 100 MHz, respectively), and HMBC NMR data, see Table 3) and tetramethylfremontin (3d, 11.0 mg, 47% yield, Rf 0.23, yellow residue): 1H NMR (acetone-d6, 400 MHz) δ 7.62 (1H, s, H-2), 7.05 (1H, s, H-5′), 6.59 (1H, s, H-2′), 6.56 (1H, d, J ) 2.3 Hz, H-8), 6.48 (1H, d, J ) 2.3 Hz, H-6), 6.05 (1H, dd, J ) 10.6, 17.6 Hz, H-2′′), 4.82 (1H, dd, J ) 1.2, 17.6 Hz, H-3′′a), 4.68 (1H, dd, J ) 1.2, 10.6 Hz, H-3′′b), 3.93 (3H, s, 7-OCH3), 3.86 (3H, s, 5-OCH3), 3.84 (3H, s, 4′-OCH3), 3.75 (3H, s, 3′-OCH3), 1.38 (3H, s, CH3-4′′), 1.33 (3H, s, CH3-5′′); 13C NMR (acetone-d6,100 MHz) δ 176.0 (C, C-4), 164.8 (C, C-7), 162.3 (C, C-5), 160.8 (C, C-9), 151.6 (CH, C-2), 150.2 (CH, C-2′′), 149.5 (C, C-4′), 147.9 (C, C-3′), 140.8 (C, C-6′), 129.7 (C, C-3), 125.1 (C, C-1′), 118.1 (CH, C-2′), 112.7 (C, C-5′), 110.5 (C, C-10), 109.5 (CH2, C-3′′), 96.7 (CH, C-6), 93.6 (CH, C-8), 56.47, 56.25, 56.21, and 56.09 (CH3 each, OCH3-5, 7, 3′, and 4′), 42.8 (C, C-1′′), 29.7 (CH3, CH3-4′′), 28.9 (CH3, CH3-5′′); HMBC correlations: H-2 to C-3, -4, -9, -1′, H-6 to C-5, -7, -8, -10, H-8 to C-6, -7, -9, -10, H-2′ to C-3, -4′, -6′, H-5′ to C-1′, -3′, -1′′, H-2′′ to C-1′′, -4′′, -5′′, H-3′′ to C-1′′, -2′′, CH3-4′′ to C-6′, -1′′, -2′′, -5′′, CH3-5′′ to C-6′, -1′′, -2′′, -4′′, OCH3-3′ to C-3′, OCH3-4′ to C-4′, OCH3-5 to C-5, OCH3-7 to C-7. Glycyrrhisoflavone (4a): brownish-white residue; UV (MeOH) λmax (log ) 261 (4.48), 292 sh (4.13) nm; IR νmax (film) 3373 (br OH), 1651, 1622, 1575, 1504, 1442, 1367, 1307, 1284, 1178, 1051, 838 cm-1;1H NMR (acetone-d6, 400 MHz) δ 13.07 (1H, br s, 5-OH), 8.09 (1H, s, H-2), 7.02 (1H, d, J ) 1.2 Hz, H-2′), 6.84 (1H, d, J ) 1.2 Hz, H-6′), 6.40 (1H, br s, H-8), 6.28 (1H, br s, H-6), 5.37 (1H, m, H-2′′), 3.37 (2H, d, J ) 7.3 Hz, H-1′′), 1.73 (3H, s, CH3-4′′), 1.70 (3H, s, CH3-5′′); 13C NMR (acetone-d6,100 MHz) δ 181.6 (C, C-4), 165.1 (C, C-7), 163.9 (C, C-5), 159.0 (C, C-9), 154.2 (CH, C-2), 144.9 (C, C-4′), 144.2 (C, C-3′), 132.3 (C, C-3′′), 128.8 (C, C-5′), 124.3 (C, C-3), 123.7 (CH, C-2′′), 122.7 (C, C-1′), 122.1 (CH, C-6′), 114.7 (CH, C-2′), 106.0 (C, C-10), 99.8 (CH, C-6), 94.4 (CH, C-8), 29.0 (CH2, C-1′′), 25.9 (CH3, CH3-4′′), 17.8 (CH3, CH3-5′′); HMBC correlations: H-2 to C-3, -4, -9, -1′, H-6 to C-5, -7, -8, -10, H-8 to C-6, -7, -9, -10, H-2′ to C-3, -3′, -4′, -6′, H-6′ to C-3, -2′, -4′, -1′′, H-1′′ to C-4′, -5′, -6′, -2′′, -3′′, H-2′′ to C-5′, 1′′, -4′′, -5′′, CH3-4′′ to C-2′′, -3′′, -5′′, CH3-5′′ to C-2′′, -3′′, -4′′; HRESIMS m/z 377.1012 [M + Na]+ (calcd for C20H18NaO6, 377.1001). Acid-Catalyzed Cyclization of Glycyrrhisoflavone. Glycyrrhisoflavone (4.0 mg, 0.01 mmol) in 1 mL of HCOOH (88%) was stirred at 90 °C for 5 h. The reaction mixture was partitioned between H2O (2 mL) and EtOAc (3 × 2 mL). After drying the organic layer, the product 4b (2.5 mg, 63% yield) was purified by HPLC (50 to 100% B in A in 50 min; tR ) 25 min): 1H NMR (acetone-d6, 400 MHz) δ 13.07 (1H, br s, 5-OH), 8.13 (1H, s, H-2), 6.93 (1H, d, J ) 2.1 Hz, H-2′), 6.84 (1H, d, J ) 2.1 Hz, H-6′), 6.41 (1H, d, J ) 1.9 Hz, H-8), 6.28 (1H, d, J ) 1.9 Hz, H-6), 2.81 (2H, t, J ) 6.7 Hz, H-1′′), 1.86 (2H, t, J ) 6.7 Hz, H-2′′), 1.35 (6H, s, CH3-4′′ and -5′′); 13C NMR (acetone-d6,100 MHz) δ 181.5 (C, C-4), 165.8 (C, C-7), 163.6 (C, C-5), 159.1 (C, C-9), 154.2 (CH, C-2), 146.6 (C, C-3′), 142.7 (C, C-4′), 124.1a (C, C-3), 123.2a (C, C-1′), 122.4 (CH, C-6′), 121.9 (C, C-5′), 114.3 (CH, C-2′), 105.8 (C, C-10), 100.0 (CH, C-6), 94.6 (CH, C-8), 75.7 (C, C-3′′), 33.5 (CH2, C-2′′), 27.0 and 27.0 (CH3 each, CH3-4′′ and -5′′), 22.9 (CH2, C-1′′) (assignments with the same superscript may be interchanged); HMBC correlations, H-2 to C-3, -4, -9, -1′, H-6 to C-5, -7, -8, -10, H-8 to C-6, -7, -9, -10, H-2′ to C-3, -3′, -4′, -6′, H-6′ to C-3, -2′, -4′, -1′′, H-1′′ to C-4′, -5′, -6′, -2′′, -3′′, H-2′′ to C-5′, 1′′, -3′′, -4′′, -5′′, CH3-4′′ and -5′′ to C-2′′, -3′′. Methylation of Glycyrrhisoflavone. Glycyrrhisoflavone (4a, 9.0 mg, 0.03 mmol) was heated to reflux with excess K2CO3 (280 mg) and excess Me2SO4 (0.5 mL, 5.3 mmol) in 9.0 mL of anhydrous acetone under 4 Å molecular sieves for 5 h. The product 4c (6.2 mg, 59% yield) was purified by HPLC (50 to 100% B in A in 50 min; tR 34 min): 1H NMR (CDCl3, 400 MHz) δ 7.77 (1H, s, H-2), 7.11 (1H, d, J ) 1.9 Hz, H-2′), 6.81 (1H, d, J ) 1.9 Hz, H-6′), 6.45 (1H, d, J ) 2.2 Hz, H-8), 6.38 (1H, d, J ) 2.2 Hz, H-6), 5.28 (1H, m, H-2′′), 3.94 (3H, s, 5-OCH3), 3.89 (3H, s, 7-OCH3), 3.88 (3H, s, 3′-OCH3), 3.82 (3H, s, 4′-OCH3), 3.36 (2H, d, J ) 7.2 Hz, H-1′′), 1.72 (3H, s, CH34′′), 1.71 (3H, s, CH3-5′′); 13C NMR (CDCl3,100 MHz) δ 175.3 (C, C-4), 163.8 (C, C-7), 161.4 (C, C-5), 159.8 (C, C-9), 152.2 (C, C-3′), 150.4 (CH, C-2), 146.8 (C, C-4′), 135.2 (C, C-5′), 132.1 (C, C-3′′), 127.7 (C, C-3), 126.2 (C, C-1′), 123.0 (CH, C-2′′), 122.1 (CH, C-6′), 111.8 (CH, C-2′), 109.9 (C, C-10), 96.2 (CH, C-6), 92.5 (CH, C-8), 60.5, 56.4, 55.8, and 55.7 (CH3 each, 5-, 7-, 3′-, 4′-OCH3), 28.6 (CH2, C-1′′), 25.8 (CH3, CH3-4′′), 17.8 (CH3, CH3-5′′); HMBC correlations, H-2 to C-3, -4, -9, -1′, H-6 to C-5, -7, -8, -10, H-8 to C-6, -7, -9, -10,
Salem and WerboVetz
H-2′ to C-3, -3′, -4′, -6′, H-6′ to C-3, -2′, -4′, -1′′, H-1′′ to C-4′, -5′, -6′, -2′′, -3′′, H-2′′ to 1′′, -4′′, -5′′, CH3-4′′ to C-2′′, -3′′, -5′′, CH3-5′′ to C-2′′, -3′′, -4′′). Calycosin (5): white residue; UV (MeOH) λmax (log ) 248 (4.44), 260 (sh 4.41), 290 (4.21) nm; IR νmax (film) 3205 (br OH), 1623, 1585, 1511, 1454, 1280, 1196, 1133, 1024, 852 cm-1; 1H, 13C, and HMBC NMR data consistent with literature values;10-12 HRESIMS m/z 307.0578 [M + Na]+ (calcd for C16H12NaO5, 307.0582). Maackiain (6): amorphous white powder; [R]D24 -33.5 (c 0.13, CHCl3); UV (MeOH) λmax (log ) 213 (4.38), 286 (3.66), 310 (3.85) nm; IR νmax (KBr) 3393 (br OH), 2927, 1624, 1474, 1456, 1347, 1320, 1186, 1144, 1122 cm-1; 1H, 13C, and HMBC NMR data consistent with literature values;14-17 HRESIMS m/z 307.0603 [M + Na]+ (calcd for C16H12NaO5, 307.0582). 3,4-Dihydroxy-8,9-methylenedioxypterocarpan (7): amorphous white powder; [R]D24 +62.2 (c 0.188, CHCl3); UV (MeOH) λmax (log ) 213 (4.66), 310 (3.85) nm; IR νmax (film) 3423 (br OH), 1625, 1476, 1459, 1338, 1206, 1144, 1052, 936 cm-1; 1H, 13C, and HMBC NMR data consistent with literature values;17,18 HRESIMS m/z 323.0521 [M + Na]+ (calcd for C16H12NaO6, 323.0532). Oleanolic acid (8): amorphous white powder; [R]D +72; UV (MeOH) λmax < 220 nm; IR νmax (KBr) 3421 (br OH), 2944, 2874, 1697 (sh), 1458, 1387, 1210, 1167, 1029 1051 cm-1; 1H, 13C, and HMBC NMR data consistent with literature values;19,39 HRESIMS m/z 479.3501 [M + Na]+ (calcd for C30H48NaO3, 479.3501). Isoliquiritigenin (9): yellow powder; UV (MeOH) λmax (log ) 242 sh (3.86), 298 sh (3.78), 371 (4.30) nm; IR νmax (KBr) 3357 (br OH), 1630, 1605, 1586, 1547, 1514, 1503, 1220, 1166, 1142;1H, 13C, and HMBC NMR data consistent with literature values;20,21 HRESIMS m/z 279.0647 [M + Na]+ (calcd for C15H12NaO4, 279.0633). Antileishmanial Assay Using Axenic Amastigotes. The antileishmanial activity of the isolated compounds was tested in vitro against L. donoVani amastigote-like parasites (WHO designation: MHOM/ SD/62/1S-CL2D) in a three-day assay using the tetrazolium dye-based CellTiter reagent (Promega) as described previously.40-42 Antitrypanosomal Assay. Compounds were tested for their activity against bloodstream-form T. b. brucei (MITat 1.2, variant 221) axenically cultured in HMI-9 medium as described by Bodley et al.43 with minor modifications. Briefly, 100 µL of late log phase parasites were incubated in 96-well plates (Costar) at an initial concentration of 105 cells/mL with or without test compounds at 37 °C in a humidified 5% CO2 atmosphere for 72 h. Ten microliters of a 20 mg/mL solution of p-nitrophenyl phosphate (prepared in 1 M NaOAc, pH 5.5, 1% TritonX-100) was then added to each well, and plates were reincubated at 37 °C as before for 6-8 h. Optical densities were then measured at 405 nm using a SpectraMax Plus microplate reader (Molecular Devices). IC50 values, the concentration of the compound that inhibited cell growth by 50% compared to untreated control, were determined with the aid of the software program SoftMax Pro (Molecular Devices). This program uses the dose-response equation y ) ((a - d)/(1 + (x/c)b)) + d, where x ) the drug concentration, y ) absorbance at 405 nm, a ) upper asymptote, b ) slope, c ) IC50, and d ) lower asymptote. This assay has the advantage of not being subject to interference by samples with a reducing potential (such as the o-catechols isolated and assayed in this study), as it depends on measuring acid phosphatase activity in surviving parasites. Cytotoxicity Assay. Cytotoxicity was evaluated against two cell lines, Vero cells and PC-3 prostate cells, obtained from the American Type Culture Collection (ATCC, Rockville, MD) as described previously.2,44 Acknowledgment. M.S. was supported by a Jane Chen fellowship and a Presidential fellowship at The Ohio State University. Additional support was provided by The Ohio State University, College of Pharmacy. We would like to thank Dr. J. Cassady for giving us the opportunity to sample his library of plant extracts for screening. We would like also to thank Dr. R. Spjut for the invaluable comments and suggestions regarding the taxonomy of P. arborescens and Dr. C. Cottrell for useful discussions and help with the NMR techniques. Supporting Information Available: The 1H and 13C NMR spectra of compounds 1a and 2 and the NMR data of compounds 5-9. This material is available free of charge via the Internet at http:// pubs.acs.org.
IsoflaVonoids from Psorothamnus arborescens
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